How to tag

Add the namespaceremoved: to all keys which are no longer relevant to the current state of an object without changing the geometry (let it a node if it was a node, a way if it was a way). You should treat all the tags on an object as a set of facts about the object, and prefix the keys of those facts which are no longer true as a result of being non existent.

Adding a note=* to explain why you just didn't delete the geometry is highly recommended, else, someone might just delete everything again.